With miniaturization of transistors, a thin film, such as an insulating film, constituting sidewall spacers (SWS) of a gate electrode, has been increasingly required to be formed at a low temperature and have a low dielectric constant and a low hydrogen fluoride (HF) wet etch rate. To meet these requirements, a silicon carbonitride (SiCN) film obtained by adding carbon (C) to a silicon nitride (SiN) film or a silicon oxycarbonitride (SiOCN) film obtained by adding oxygen (O) to the SiN film may be used as an insulating film. Since these insulating films need high step coverage characteristics, there have been many cases in which the insulating films are formed using an alternate supply method of alternately supplying process gases (for example, refer to Patent Document 1).